A ceramic substrate is excellent in heat resistance and moisture resistance, and also has satisfactory frequency characteristic in a high frequency circuit. Therefore, the ceramic substrate is used as an RF module for mobile devices, a substrate for power LEDs utilizing heat dissipation properties, and a substrate for LED backlights of liquid crystals. A ceramic substrate composite with a conductor pattern formed on the ceramic substrate is used as a core substrate of a build-up substrate. A build-up layer to be provided on the ceramic substrate composite is formed using thin film formation technology such as a sputtering method, a CVD method, or a sol-gel method.
The build-up substrate is likely to be influenced by a state of contact between a surface of the ceramic substrate composite and the build-up layer. Namely, it is impossible to sufficiently ensure the state of contact between the surface of the ceramic substrate composite and the build-up layer if the ceramic substrate composite does not have a sufficiently flat surface. Therefore, there arises a problem that it is impossible to obtain characteristics of the build-up substrate in a stable manner and to sufficiently ensure insulation between the ceramic substrate composite and the build-up layer.
In order to solve such problem, the surface of the ceramic substrate composite is subject to glass coating to thereby flatten the surface of the ceramic substrate composite (Patent Document 1 and Patent Document 2). The surface of the ceramic substrate composite subjected to glass coating is polished to thereby expose the conductor pattern formed on the ceramic substrate.
Patent Document 1: JP 2005-136396 A
Patent Document 2: JP 2010-98291 A